CyanogenMod already has nightlies available for the T-Mobile and Canadian (mine) GS4's since they're unlocked. So I have GS4 hardware with Nexus 4 software =)
I'm currently still on the stock ROM albeit rooted and all the Samsung bloat frozen using TiBack. All the S-etc. and Air-XYZ crap has been disabled too. Multi-window can stay; I'll miss it when I go to CyanogenMod. Much smoother.
I've been using my GS4 naked for the past couple days while waiting for my Spigen Neo Hybrid case to come in. Being VERY careful because I might cry if I dropped and broke it haha. This thing is amazingly light too (although for some reason, lightness is no longer considered a premium quality like it was 10 years ago).
I've been comparing it to a friend's GS3 and wow they really do look alike. On the other hand, the feel in hand is noticeably different if you grip it and apply some force. People have complained about creaking in the GS3 (and I've noticed it in my Note 1) but it's absent for the most part in the GS4. However, creaks sometimes develop over time so I'll see what it's like down the road in 3 months.
Screen is day and night compared to previous AMOLEDs I've owned (Nexus One, Note 1). I owned the One X instead of the GS3 and always ribbed my friend about his way less sharp GS3. I can't see the effect now even when looking for it (and I know how to look for it). I suppose having the Red and Blue subpixels individually on par with the iPhone 5's screen means you'd need to be closer than 8 inches to see the Pentile effect at all. I bought a phone with large screen so I wouldn't have to squint while I peer at it from point blank =)
Camera is only par for indoor shots. It's quite capable for static shots (if you have a steady hand) but will not capture motion as well as the One can. Outdoors though, the camera is brilliant.
Here's some stuff I snapped over the weekend. Every one was using Full Auto, no Burst (so I didn't just take a bunch at once and choose the best one). I've noticed Sport mode bumps up the ISO and doubles the shutter time. Pictures which would use ISO-50, 1/1300s shutter would be ISO-100, 1/2600s shutter instead. I'm happy with the outdoor results, there's a bit too much oversharpening done in the filters but the halos are only really visible at 100%. Click to open full size.
My bike wheel after I set it spinning.
Chose a dandelion to see the captured details.
The red turned out nicely.
Cropped from original. Not sports mode.
Cropped for the detailed part.
Cropped and not sports mode to capture the motion.
One other thing that I've found pleasant about the GS4 is that it can reliably lock GPS indoors (except in deep in concrete buildings or metal boxes). None of my previous phones even came close to doing that.